The invention relates to security devices containing zero-order diffractive microstructures. Such devices may be used as security devices in the fields of authentication, identification and security.
The production of zero-order diffractive microstructures having special colour effects—for example, colour change upon tilting and/or rotation—for use as security devices in a variety of applications like (but not restricted to) banknotes, credit cards, passports, tickets, document security, anti-counterfeiting, brand protection and the like is known. The devices can be in the form of hot or cold transferable labels, adhesive tags and the like. They can significantly decrease the possibility of counterfeiting compared to state of the art security devices possessing security printing techniques, optically variable devices (OVDs) like optically variable inks (OVI) or diffractive optically variable image devices (DOVIDs), UV/IR fluorescent dyes, magnetic stripes etc.
It is known to use DOVIDs like holograms for anti-counterfeiting of banknotes or credit cards. Further magnetic codes or fluorescent dyes are often used to proof the originality of items. Unfortunately it is already possible to produce high quality counterfeited versions of devices using all those techniques. Additionally, DOVIDs possess only a low level of security, as non-experts generally do not know what the holographic image looks like. Therefore there is a need for novel security devices that are more difficult to counterfeit.
OVIs, as disclosed in the U.S. Pat. No. 4,705,356, provide higher level of security, as it is easier for non-experts to observe a colour change than a complex image. Although OVI's are also difficult to manufacture, and therefore seem to be secure, their effect can be closely mimicked with colour-shifting inks that are used for decorative purposes and are commercially available from several companies (for example Colour Shift). This decreases the value of OVIs as anti-counterfeiting tool.
In the U.S. Pat. No. 4,484,797 colour filter with zero-order microstructures are described for use as authenticating devices. When evenly illuminated with non-polarized, polychromatic light such devices show unique colour effects upon rotation and therefore can be clearly identified. The possibilities for varying the colour effect are limited (see M. T. Gale “Zero-Order Grating Micro-structures” in R. L. van Renesse, Optical Document Security, 2nd Ed., pp. 277) and no individualization of the devices is claimed.
The WO 03/059643 also describes very similar zero-order diffractive gratings for use in security elements. The elements have the same drawbacks as the filters in the U.S. Pat. No. 4,484,797.